1. Field of the Invention
This invention generally relates to processing of materials, including growth or deposition of the materials, and also to removal of materials. More specifically, the invention relates to a method and apparatus for locally raising the temperature of a material in order to facilitate chemical reactions or changes in physical state related to processing of the material by using an electrode tip, such as an electron emitter tip, to apply a controlled succession of thermal annealing spikes (hereinafter referred to as thermal spikes) or shockwaves of varying energy through a growth or removal medium (hereinafter referred to as a growth medium, even though the medium could also be used in processes involving etching or cleaning and no xe2x80x9cgrowthxe2x80x9d). The scale of the thermal spikes or shockwaves, and the area of the material affected by the resulting energy transfer, is on the order of a few nanometers to several hundred micrometers, and the duration of the thermal spikes or shockwaves ranges from a few picoseconds to several hundred nanoseconds. The growth medium may be a cryogenic liquid, although it is within the scope of the invention to use other growth media, including liquids, solids, gases in critical or non-critical state, and mixtures of liquids and solids, solids and gases, and liquids and gases.
The method and apparatus of the invention may be used for a variety of industrial applications and manufacturing processes, including deposition and/or growth of thick or thin film crystalline or non-crystalline materials, etching or cleaning of materials, and formation of nanostructures.
2. Description of Related Art
A. Introduction
The problem addressed by the present invention is the problem, common in the field of materials processing, of how to add energy during processing of a material in order to speed-up chemical reactions or processes, overcome energy barriers, or otherwise improve processing efficiency or product quality, without generating defects or damaging in the material, and/or without interfering with other reactions or processes necessary to provide an acceptable end product.
The oldest and simplest way to apply energy to a material during processing is simply to process the material in a high temperature environment as in, by way of example, physical vapor deposition (PVD), chemical vapor deposition (CVD), or metalorganic chemical vapor deposition (MOCVD). Alternatively, energy may be applied directly to a material being deposited or transferred, as in sputter deposition and various etching methods, by using shockwaves as in explosive bonding methods, or by direct application of energy using lasers or radiation. In all such materials processing methods, the objective has always been to transfer the necessary energy in the most efficient manner while minimizing any damage that might occur as a result of the energy transfer.
The present invention also addresses the problem of energy transfer efficiency and damage mitigation, but utilizes a mechanism different from all other known materials processing methods and apparatus, namely the application, in the presence of a growth medium, of a controlled succession of thermal spikes or shockwaves of varying energy to a nanoscale area of the material.
The term xe2x80x9cnanoscalexe2x80x9d as used herein refers to dimensions on the order of less than one nanometer (including atomic dimensions of approximately 0.1 to 0.15 nm) to several tens of micrometers, as opposed to the dimensions of the non-localized high energy shockwaves produced by spark discharges for the purpose of vaporizing materials in order to facilitate binding of coatings to a surface, as described for example in U.S. Pat. No. 3,663,788. The use of shock waves on a macroscopic scale is a variation of the explosive bonding technique used to join otherwise incompatible metals, which is fundamentally different than the much more controlled application of energy provided by the present invention.
The most relevant prior art known to the inventor, which shares with the present invention localization of the energy transfer, and the use of a cryogenic growth medium (optional in the present invention), is the xe2x80x9ccryogenic furnacexe2x80x9d technique disclosed in U.S. Pat. No. 3,720,598. According to this technique, an oscillating Josephson junction having xe2x80x9cextremely small dimensionsxe2x80x9d is formed by spark erosion between capacitor electrodes made up of the materials to be vaporized, thereby concentrating as much energy as possible on a small area.
The present invention shares with the cryogenic furnace concept the temporal and spatial localization of energy applied to a material for the purpose of xe2x80x9cestablishing chemical and physical state in materialsxe2x80x9d (col. 1, lines 14-27 of U.S. Pat. No. 3,720,598), and in particular to facilitate growth or removal of materials, as well as the use (in a preferred embodiment of the present invention) of a cryogenic medium through which the energy is applied, but is distinguishable in a number of ways:
instead of using a plasma arc discharge to temporarily vaporize the medium and material being processed, the present invention uses electron emission (either from an electron emitter tip, or from the workpiece or growth medium in case the polarity of the emitter tip is reversed) to generate thermal spikes or shockwaves that propagate in the medium in order to enable a more controllable energy transfer, eliminating the high energy plasma ions inherent in plasma discharge arrangements;
The impulses are controlled to provide a succession of spikes of varying energy rather than a steady state or oscillating field;
The size of the area affected by the discharge is reduced even further than in the cryogenic furnace technique to nanometer or atomic scale, thereby reducing the overall amount of energy that needs to be supplied to achieve a desired local temperature; and
The apparatus in which the growth or removal of materials takes place is adapted to facilitate insertion and removal of materials from the growth medium, cleaning of the growth medium, as well as insertion and removal of the substrate on which growth occurs or from which material is to be removed, so as to enable use of the system in industrial manufacturing processes.
These differences are critical to the practicality of the present invention relative to the technique disclosed in U.S. Pat. No. 3,720,598. While capable of delivering high energy levels to a small area, the cryogenic furnace technique described in U.S. Pat. No. 3,720,598 ultimately proved impractical for manufacturing purposes because of the inability to prevent destruction of an unacceptably high percentage of the grown material by the high energy tail in the distribution of ions created by the plasma arc. Furthermore, alternative techniques that were eventually implemented in the years following the originally cryogenic furnace proposal, such as electron synchrotron radiation, laser heating, and rapid thermal annealing, while more controllable and less destructive, are capable of delivering only a relatively small amount of energy over a relatively large area, barely sufficient to break chemical bonds and improve mobility on growing surfaces. To date, the most promising of these methods is electron synchrotron radiation, but this method requires equipment costing a minimum of $100 Million, and is not readily available or adaptable to ordinary manufacturing.
While the present invention shares with several of the above-mentioned prior arrangements the concept of applying spatially and temporally localized bursts of energy to a material in order to change its physical or chemical state and facilitate material growth, the manner of energy delivery is fundamentally different, involving the propagation in the growth medium, and in particular a cryogenic growth medium, of nanoscale bursts of energy in an arrangement adapted for mass processing of the materials to be grown or otherwise altered or formed.
The method and apparatus of the invention may be used as a replacement for a variety of conventional systems, including CVD and MOCVD, as well as sputter deposition, molecular beam epitaxy (MBE), electrical plating in electrochemical solution, and other methods too numerous to list. Although specific examples of materials are set forth below, the invention is not intended to be limited to specific materials or groups of materials, but rather is intended to apply to any materials capable of being grown in bulk form or on a substrate, or of being removed from a substrate, in an appropriate growth medium, using shock or heat impulses that propagate through the cryogenic medium in order to apply energy or heat to the medium on a nanometer scale.
It is accordingly a first objective of the invention to provide a method of applying energy to a material for the purpose of facilitating chemical reactions or processes related to growth or removal of an area of material, without damaging the material due to the high temperatures normally required to facilitate such chemical reactions or processes.
It is a second objective of the invention to provide a method of facilitating chemical reactions or processes related to growth or removal of an area of material by temporally and spatially localized application of energy, without the problems of high energy tails inherent in plasma arc discharge methods.
It is a third objective of the invention to provide a method of energy enhanced material growth or removal which utilizes readily available equipment suitable for use in large-scale commercial manufacturing.
It is a fourth objective of the invention to provide a method of more efficiently growing nanostructures of various shapes and for a variety of purposes.
It is a fifth objective of the invention to provide apparatus for applying energy to a material for the purpose of facilitating chemical reactions or processes related to growth or removal of an area of material, without damaging the material due to the high temperatures normally required to facilitate such chemical reactions or processes, and without the problems of high energy tails inherent in plasma arc discharge methods.
It is a sixth objective of the invention to provide apparatus adapted to facilitate insertion and removal of materials from a medium in which growth and/or a material removal process such as etching is to occur, cleaning of the cryogenic growth medium, as well as insertion and removal of the substrate on which growth occurs or from which material is to be removed, so as to enable use of the apparatus in industrial manufacturing processes.
It is a seventh objective of the invention to provide apparatus capable of performing multiple processing steps, including growth of materials on a workpiece followed by cleaning and/or etching of one or more materials, utilizing a single cell divided into multiple subcells or sub-areas, and a single controller, without requiring removal and transport of the workpiece from the apparatus between processing steps.
It is an eighth objective of the invention to provide an etch-transport-growth reaction path that enables recycling of etched materials back into the growth cell.
These objectives are achieved, in accordance with the principles of a preferred embodiment of the invention, by providing a method of applying energy to a substrate or material on which additional material is to be grown, or from which material is to be removed, by transmitting heat or shock impulses to the substrate or material through a cryogenic medium. The heat or shock impulses are applied by a nanometer scale electrode tip immersed in the growth medium, which in the illustrated examples is a cryogenic liquid, to which is applied fast, variable, voltage pulses of on the order of picoseconds to hundreds of nanoseconds. The anode to which electrons are transmitted may either be a discrete element or a portion of the workpiece, and may consist of an individual scannable electrode tip or an array of electrode tips. In the latter case, the electrons are drawn from the electrode tip into the liquid and accelerated until they hit a specific target area of the workpiece or substrate being processed, while an individual electrode tip can be used to transmit shockwaves parallel to the workpiece and thereby indirectly transmit energy to a larger area of the workpiece.
In the case of a material growth apparatus suitable for use in industrial processes, such as manufacture of thin film or semiconductor devices, the apparatus of the invention may, according to a preferred embodiment of the invention, include any or all of the following features:
a growth chamber;
a mechanism for introducing growth material into the growth chamber;
a mechanism for cleaning the growth medium to remove growth material and contaminants from the growth chamber;
a single electrode tip or array of tips arranged to operate on a nanoscale;
a controller for controlling a discharge from the single tip or array of tips;
a regulator for controlling temperature and pressure of cryogenic liquid in the chamber;
a mechanism for circulating the growth medium through the growth chamber; and
provision of at least one chemically inert working cell surface with very low physisorbtion energies.
Since the same basic structure may be used for both growth and removal of a material or materials from the substrate or workpiece, the growth chamber of an especially preferred embodiment of the invention may utilize a single cell divided into multiple subcells or sub-areas for sequentially processing the substrate or workpiece by deposition of materials, followed by etching or cleaning of the resulting product. In a particularly advantageous implementation of the preferred embodiments of the invention, growth, etching, and cleaning all occur in a single growth chamber divided into multiple cells, permitting recycling of etched materials.
In the illustrated examples, the growth medium in which growth or removal of materials occurs is a cryogenic liquid which is preferably chemically inert. Examples include Argon and related noble elements, with the liquid serving to (i) act as a local heating medium on a scale of a few nanometers to several hundred micrometers, (ii) lower the growth temperature and slow down chemical reactions or kinetics, and (iii) act as a carrier for growth or removed materials to and/or from the growth zone.
Alternatively, depending on the density and/or pressure of the growth medium and on the tip distance, the growth medium may serve primarily as a carrier for growth or removed materials and/or to keep a growth species in place following transfer of energy to the workpiece.
Examples of materials to which the present invention can be applied are, GaN, which is used to form blue lasers, conventionally grown at T=600-800xc2x0 C. using the metalorganic chemical vapor deposition (MOCVD) method, and Al2O3 hard coating films conventionally grown by CVD at temperatures of 1000-1100xc2x0 C. in order to obtain the hard crystalline xcex1 and xcex2 phases of Al2O3, although the invention is not to be limited to any specific materials or group of materials.
In addition to the above-described processing of materials by growth, etching, cleaning, and/or other processing steps, the method and apparatus of the invention may be used in conjunction with auxiliary processing methods and apparatus for facilitating the processing. By way of example but not limitation, processing of a material using the method and apparatus of the invention may be assisted by an optional photon source of sufficient energy to lower reaction barriers or and/or to break or weaken chemical bonds in order to further facilitate the growth, etching, cleaning, and or other processing of the material.